So much so that, on a cyclically adjusted PE (the only PE that matters), stocks had become 30% overvalued.

So how are stock values looking now that the market is crashing again?

Better, but still expensive.

Measured using our favorite valuation technique, Professor Shiller's cyclically adjusted PE analysis, the S&P 500 now has a PE of 19X. That's down from 23X just a few weeks ago. But it's still considerably above the long-term average (1880-2010) of about 16X.

Check out the chart below, from Professor Shiller's web site. The blue line is the cyclically adjusted PE ratio (CAPE) for the last 130 years. (The cyclically adjusted PE mutes the impact of the business cycle by averaging 10 years worth of earnings. This reduces the misleadingly low PEs you get at peak profit margins, like the ones in 2007, and the misleadingly high ones at trough profit margins, such as the ones we had last year).

Note a few things:

The long-term average for the cyclically adjusted PE is about 16X.

Stocks have spent vast periods above the average and vast periods below it, usually in multi-decade cycles

We've just descended from the longest period of extreme overvaluation in history, suggesting (to us, anyway) that the next multi-decade cycle is likely to be below average

At today's level, 10700 on the S&P, stocks are trading at a 19X CAPE, about 15%-20% above the long-term average

Now, you can also unfortunately see from the chart that valuation doesn't tell you anything about what will happen next. As the blue line shows, just because stocks are overvalued today doesn't mean they won't just get more overvalued. And they can stay even more overvalued for a decade or more.

But what the apparent overvaluation does tell you--or, at least, has told you in the past--is that your future long-term returns will likely be below average. There's a strong correlation between starting valuations and ending returns (high valuations lead to low returns and low valuations lead to high returns). And today's valuations can now be described as "high." (Not extreme, but high.)

Today's PE also clearly shows that stocks aren't "cheap" and that today's crash is not necessarily a "buying opportunity." Fair value for stocks is about 900 on the S&P 500. So it certainly wouldn't be surprising to see stocks trade back to that level and possibly below it.

Yes, you can argue that "it's different this time." You can argue that, since stocks have traded at an average CAPE of more than 20X for the past two decades, we're in a new normal. And you might be right. But they don't call "it's different this time" the "four most expensive words in the English language" for nothing.

You can also argue that "interest rates are low, so P/Es should be high." That argument is in vogue right now, because there's been an inverse correlation between P/Es and interest rates for the last couple of decades.

But take a look at the RED line in Professor Shiller's chart. The red line is interest rates. As you can see, if you go back more than a couple of decades, there's not much correlation. (In fact, as the great UK economist Andrew Smithers has observed, there's none.)